This invention relates to an improved bearing assembly for use with a rotary table of the type used on heavy machinery, and it particularly relates to an improved air bearing assembly wherein the effective air bearing lift area is greater than conventional prior art air bearings or airings.
Rotary tables are commonly used with various types of heavy machinery. One such use of rotary tables is in machines wherein each of a plurality of workpieces is moved by indexing from station to station for one or more machining operations thereon, such as drilling, boring, facing, tapping and the like. In such equipment, the workpieces are mounted on fixtures which are arcuately spaced from each other and fixed to the upper surface of a rotary table. The table is rotatably indexed intermittently so that the workpieces are moved under the tools carried on the head of the machine, where the various machining operations are carried out.
Typically, a rotary table is rotatably supported on a base by an antifriction thrust bearing, such as a ball bearing or a roller bearing. Such tables and thereby the bearings therefore are large. For example, it is common to have a 100 inch diameter for the working surface of the rotary table. Such machines are important production machines and any unscheduled down time for such machines as a result of failure of any part of the machines can be extremely costly because of the lost production time. One critically important part of such equipment, which is not only expensive but also has a long delivery term, is the antifriction bearing used to rotatably support the table on the base.
Because of inherent disadvantages of antifriction roller or ball thrust bearings, these bearings have been replaced by air bearings or "airings" wherein air pressure is used to form a bearing between the table and the base so that the rotatable table, in effect, "floats" above the upper surface of the base. These bearings have generally comprised a fixed annular or ring like member having a substantially rectangular cross-sectional shape. The lower surface of the air bearing member rests on the base while the upper surface thereof is positioned below the rotary table. Air pressure is applied through the bearing and air pressure is imparted between the upper surface of the bearing and the lower surface of the rotary table so that air pressure acting on the available surface generates the desired lift force to lift the table which, in effect, floats on air. One significant limitation of these air bearings is that there is only limited area available which may function as the bearing lift area. Therefore, the air pressure may have to be increased so as to support the larger, heavier tables. If the air pressure has to be increased above the available plant air pressure to generate sufficient lift force, the advantage of an air bearing may be lost or the use of an air bearing may be totally unfeasible.